This section provides information helpful in understanding the invention but that is not necessarily prior art.
Modern golf balls are known to have solid cores made from a variety of materials. The material making up a golf ball core may affect the golf ball's performance characteristics in several ways. For example, the selection of the material for use as a golf ball core may affect the golf ball's coefficient of restitution, initial velocity off the tee, feel, durability over time, and other properties.
Suitable known core materials include thermoset materials, such as rubber, styrene butadiene, polybutadiene, isoprene, polyisoprene, and trans-isoprene. Butadiene is a commonly used core material, but water absorption is known to result in loss of distance. A number of patents describe moisture barrier layers in golf balls to prevent this from happening, including Sullivan et al., U.S. Pat. No. 8,303,436 and Hogge et al., U.S. Pat. No. 7,357,733.
Core materials also include thermoplastic materials. For example, Rajagopalan et al. U.S. Pat. No. 6,756,436, entitled “Golf Balls Comprising Highly-Neutralized Acid Polymers,” which is incorporated herein by reference, discloses golf balls having highly neutralized acid polymer cores. The highly neutralized polymer may be mixed with a second polymer to change the compression of the core. The second polymer may be ionomeric copolymers and terpolymers, ionomer precursors, thermoplastics, thermoplastic elastomers, polybutadiene rubber, balata, grafted metallocene-catalyzed polymers, non-grafted metallocene-catalyzed polymers, single-site polymers, high-crystalline acid polymers, cationic ionomers, and mixtures of these.
Polyurethane materials have been used in golf balls, for example as described in Michalewich et al, U.S. Pat. Pub. No. 2011/0081492, “Methods of Curing Polyurethane Prepolymers for Golf Balls” and filed on Oct. 5, 2009, incorporated herein by reference, which discloses a golf ball having a cover layer made from a polyurethane pre-polymer. The polyurethane pre-polymer system is formed from a reaction mixture having an isocyanate index of 1.20 or more. The cover is subsequently exposed to moisture in order to effect full curing of an isocyanate-functional prep-polymer. The cover is used over a conventional rubber core to improve the golf ball cover's durability and resistance to tears and cuts.
Molitor, U.S. Pat. No. 4,726,590, entitled “High Coefficient Golf Ball Core” and incorporated herein by reference, discloses one-piece golf ball cores with improved coefficients of restitution. The cores are made of rubbers such as polybutadiene crosslinked with an ethylenically unsaturated metal salt such as zinc mono- or diacrylate or methacrylate and a polyfunctional isocyanate such as 4,4′-diphenylmethane diisocyanate. The polyfunctional isocyanate is said to harden the core, decreasing compression values and increasing COR.
There remains a need in the art for new core material formulations that improve physical properties of the core and the golf ball.